The present invention relates to a refractory plate which is used in the production by firing of refractories, fine ceramics, china and ceramics for electronic parts etc.
In the firing of the above described products, it is popular to use tool bricks called firing base or shelf plate. Hereinafter they will be referred to as shelf plate.
The properties that shelf plate are required to have are:
(1) No reaction with the product
(2) High crack resistance after repeated use
(Merit in cost saving)
(3) No deformation
(4) High heat efficiency, etc.
It is therefore necessary to select shelf plate material which is compatible with the quality of the fired products. As for the materials of shelf plate, oxides such as Al.sub.2 O.sub.3 --SiO.sub.2, Al.sub.2 O.sub.3 ZrO.sub.2 and non-oxides such as SiC are the main materials at present, but there is no shelf plate which has all-round properties.
That is, Al.sub.2 O.sub.3 --SiO.sub.2 shelf plate is limited in use because it may react with a product and may deform while firing. In the case of Al.sub.2 O.sub.3 shelf plate, high temperature firing is necessary, so it is expensive and also it may react with certain products.
In the case of ZrO.sub.2 shelf plate there is no reaction with product and no deformation occurs, but it has drawbacks such as being heavy to handle because of its high specific density, it is high priced, has low thermal conductivity and begins to crumble after repeated use due to structural brittleness caused by ZrO.sub.2 transformation.
In the case of SiC, it is resistant to deformation but it needs expensive coating to prevent oxidation aging and reaction with products.
To solve the above described problems, in the case of producing Al.sub.2 O.sub.3 shelf plate, a three layered shelf plate with a center layer of SiC having excellent creep resistance being sandwiched by two Al.sub.2 O.sub.3 layers is being produced.
However a three layered shelf plate is expensive, and it is difficult to make three uniform layers. Furthermore after repeated use, cracks like peeling between layers may occur so it is not an effective solution for the above described problems.
Another convential method, Japanese Utility Model Open No. 61-192300 (1986) discloses a ceramic sheet-lined refractory vessel, in which a ceramic sheet of 0.1 to 5.0 mm in thickness is lined onto an inner plane surface or back plane surface of a refractory vessel in single or double layers by pressing or pasting them together and then firing them to make an integrated refractory vessel.
However this ceramic sheet is made using a paper manufacturing method, and if a green ceramic sheet is applied to a refractory vessel it will shrink at least 15% near the firing temperature, and cracks will also appear in the ceramic sheet so it is difficult to use such sheets for a refractory vessel. Also, if a rigid sheet of fired material is used it is difficult to fit it to a refractory vessel to be used as a substrate and if it were to be successfully fit to a refractory vessel, cracks or peeling will occur between the sheet and the refractory vessel after repeated use at the practical temperatures due to the differences in thermal expansion between the ceramic sheet and refractory vessel.
Therefore it is difficult for such sheets to stand up to use as a shelf plate which goes through repeated temperature changes.